RFR: 7030853: JDK 7 Serializable Swing classes not compatible with JDK 6

[Prasanta Sadhukhan]

The serialized form of the following classes in JDK 7 builds are not compatible with prior versions of Java:
    javax.swing.text.html.parser.ParserDelegator
    javax.swing.LayoutComparator
They need to be declared as not compatible between releases in the API documentation, as is the case for some other serializable Swing classes.

Since It is widely known that serialized Swing classes are not comptible between major releases, we just didn't add the warning at that couple classes.
Added the warning..

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Commit messages:
 - 7030853: JDK 7 Serializable Swing classes not compatible with JDK 6

Changes: https://git.openjdk.org/jdk/pull/11565/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11565&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7030853
  Stats: 13 lines in 2 files changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11565.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11565/head:pull/11565

PR: https://git.openjdk.org/jdk/pull/11565